How Does an MRI Work?

How Does an MRI Work?

How Does an MRI Work?

Modern medicine involves some true technological wonders — some that we may take for granted.

You're probably familiar with equipment like magnetic resonance imaging (MRI) machines, but you probably don't know how they work. Understanding how these commonplace yet impressive tools work can give you a new appreciation for medical science.

Even if you've never had an MRI exam, you've probably seen them before in movies or TV. Every year in England, there are roughly 62 MRI exams per 1,000 people, so these are reasonably common tests. So how do they work?

What an MRI Machine Does

MRI machines offer a method of scanning the human body. When you go to the hospital to have an MRI scan, you'll lie on a bed that will move into a large circular scanner. During the scan, the machine will use a magnetic field and radio waves to create images of the inside of your body.

Like an X-ray, an MRI shows doctors what's going on under your skin, but they work differently. MRIs highlight some things other types of scans may not display, like soft tissue movement. You might've seen MRI images in shows or movies depicting characters' brain function.

Doctors can use MRIs to examine almost any body part, from your brain to your bones. Some common conditions MRIs can address include:

  • Knee and shoulder injuries
  • Aneurysms
  • Tumours
  • Heart problems
  • Diseases of the liver
  • Pelvic pain
  • Breast cancer

Some researchers use MRIs to learn more about how the human brain works. By seeing how different areas of the brain react in varying circumstances, they can get a better idea of what each part does. 

The Science Behind MRIs

To understand how an MRI works, you first need to know about your body. As you probably already know, your body is mostly water, and water consists of hydrogen and oxygen. The protons at the center of each hydrogen molecule are sensitive to magnetic fields.

An MRI machine generates a strong magnetic field, between 0.5 and 3.0 Teslas, that aligns with the hydrogen protons in your body. After the magnets activate, the machine sends bursts of radio waves that interrupt the proton alignment. When your protons realign after this burst, it creates a radio signal that receivers pick up.

By using these signals, a computer can create a digital image of whatever you're scanning. The protons in different types of tissue realign at varying speeds, meaning the scan can distinguish between them. That's also how an MRI can highlight things like tumours and cysts.

MRI machines also use coolers to control the magnets' temperature, ensuring accurate results. If the magnets get too hot, this could lead to warped images that wouldn't serve much use. 

Types of MRI Exams

There are several types of MRI scans, each serving a unique purpose. Some of the most common kinds of MRI are:

  • Functional MRI (fMRI)
  • Cardiac MRI
  • Breast scans
  • Magnetic Resonance Angiography (MRA)
  • Magnetic Resonance Venography (MRV)

An fMRI measures brain activity by looking at how blood flows to different parts of your brain. Most of the time, this involves the patient performing various actions while in the scanner. These tests help doctors treat people with conditions like Alzheimer's disease and are useful research tools.

As you might've guessed from the name, cardiac MRIs focus on the heart. These allow doctors to diagnose different heart conditions and decide upon the right treatment. Similarly, breast scans enable hospital staff to examine patients who may have breast cancer.

MRAs combine traditional MRIs with intravenous (IV) contrast dyes, which highlight blood vessels. Before the scan, doctors will inject the dye into a patient so their veins and arteries will show up clearly on the results. This process can help diagnose any issues with blood flow a patient may have.

MRVs are similar to MRAs but focus specifically on veins, not arteries. The process is almost identical to an MRA, but doctors will inject the IV contrast dye into a vein, not just any blood vessel.

How Safe Is an MRI?

For the most part, an MRI scan is completely safe, but there are some exceptions. Since these machines use potent magnets, anyone with a metal implant may not be able to get an MRI. Pacemakers, artificial joints and cochlear implants could all pose a risk under these magnetic fields.

Some types of tattoo ink contain traces of metal, but most people with tattoos can get an MRI safely. Excluding the risks metal parts may present, the magnets within these machines are harmless to humans. They may even be safe for pregnant women, although doctors don't recommend getting an MRI if you're pregnant.

When the magnets turn on and off, they create a loud noise that may bother some people. Hospitals will provide you with hearing protection before starting the procedure to avoid any problems related to the sound. Some people may find the scanner uncomfortable, especially if they have claustrophobia, but mild sedatives are available to help with that.

All things considered, MRI scans are some of the safest medical procedures out there. Since they don't use ionizing radiation like an X-ray or computerized tomography (CT) scan, they're undoubtedly safer than other imaging techniques.

MRIs Can Save Lives

If you've never had an MRI before, getting one can seem a little intimidating. When you look at the science behind these procedures, though, you can see that you don't have to worry about anything. They may be a little uncomfortable, but they're entirely safe and provide a useful service.

Without MRIs, diagnosing or addressing some conditions could be much more challenging. These machines also enable invaluable medical research. As commonplace as they may be, MRIs are truly remarkable.

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  • Linda Crook

    Excellent !

  • Ryan Parrott

    Well explained

  • Jack Bailey

    Interesting to know.. Cheers Megan

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Megan Ray Nichols

Science Expert

Megan Ray Nichols is a science writer by day & an amateur astronomer by night (at least when the weather cooperates). Megan is the editor of Schooled By Science, a blog dedicated to making science understandable to those without a science degree. She also regularly contributes to Smart Data Collective, Real Clear Science, and Industry Today. Subscribe to Schooled By Science for the latest news.

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